1. Field of the Invention
This invention relates to a method and apparatus for inhibiting obstruction of optical transmission through a semiconductor etch process chamber window or viewport by deposition of polymeric materials on the window. More particularly, it relates to a method and apparatus for selectively heating the window to selectively inhibit such deposition.
2. Description of the Related Art
In the processing of semiconductor wafers to form integrated circuit structures, the selectively removal of materials is carried out by etching. Such etch removal processing may involve a wet chemistry etch, but more likely will involve a dry etch in a sealed plasma etching chamber, which may be under vacuum, and wherein the progress of the etch will be monitored optically through a viewing window or "viewport". Typically such optical monitoring involves monitoring certain known emission lines, e.g., the 288 nm. CO.sub.2 line or the 484 nm. CO line, through a window provided in a chamber wall of the etch chamber comprising an optically transparent material.
Such plasma etch processing usually also involves the formation and deposition of polymeric materials, e.g., carbon/hydrogen products, in the chamber which deposit on the wafer and are consumed during the course of the etch process. Such polymeric formation, deposition, and consumption of same on selective surfaces of the wafer, are of great benefit in certain etch processes to assist in maintaining desired anisotropic properties of the dry etch.
However, the deposition of such polymeric materials is usually not limited to deposition on the wafer, but also occurs on all internal surfaces of the chamber as well to varying degrees. Chamber surfaces closest to the gas inlets, surfaces in direct line of sight of such inlets, and surfaces at lower temperatures (than the wafer temperature) are coated quicker, and heavier, than other surfaces, by a phenomenon known as thermophoresis meaning "being carried by heat".
When the etch chamber is equipped with one or more viewing windows or viewports for optical monitoring of the progress of the etch by monitoring the composition of the excited species adjacent to the wafer surfaces, such window surfaces also become coated with this polymeric material and/or other etch residues. Such depositions cloud the window causing instrumentation monitoring of the plasma to either not perform or malfunction.
This problem can be seen by comparing the emission spectra shown in FIGS. 1 and 2. FIG. 1 shows the emission spectra for a new or clean viewport or window, while FIG. 2 shows the emission spectra seen through the same window after about 230 minutes of use which shows the effects of such depositions thereon. In particular, it will be noted that the wavelength region of about 250 nm. to 450 nm., the very region where the emission lines of interest are monitored, appears to be the most severely impacted by the depositions on the surface of the window.
This problem of deposition of etch residues on the surfaces of the etch chamber, including the window, has been addressed in the prior art. Attempts to reduce such depositions on viewing windows have previously included the interposing of fine mesh screens in front of the window, placing the window at the end of a long tube in the etch chamber fabricated of nonconducting material, and by heating the window.
However, these prior art attempts to solve the problem are not without their own problems. Screens and wires obstruct the view and require periodic cleaning, long tubes prevent a wide field of vision, and windows equipped with perimeter heaters still become coated at the slightly cooler center. In fact, it has been noted that some migration of the deposited materials toward such cooler surfaces occurs when heaters are employed to heat the window surface.
In addition, the use of heated windows in combination with insulation materials to prevent thermal coupling between the heated window and the etch chamber resulted in deterioration of the insulation materials causing debris, noxious odors, and most importantly, sublimation of outgassing products of the insulation onto the outer surface of the window resulting in film formation which interfered with plasma light transmission and optical monitoring of the etch.
It would, therefore, be desirable to provide a method and apparatus for inhibiting the obstruction of optical transmission through the viewport or window of a semiconductor etch process chamber by either deposition of polymeric materials, etch byproducts, or residues on the inner surface of the window or deposition of insulation byproducts on the exterior surface of the window.